Photodefinable polyimides are coming into increased use in the manufacture of microelectronic devices, driven by the reduced number of processing steps necessary to pattern polyimide coatings. Applications include interlayer dielectrics on integrated circuits and for interconnects, passivation layers, thermal and mechanical stress buffers in packaging, alpha particle barriers on memory devices, and ion implantation and dry etching masks.
Current commercial products include negative-working systems based on free-radical crosslinking of poly(amic methacrylate esters) or poly(amic methacrylate salts). Although these allow for deposition of relatively thick films, they must be solvent developed and suffer from large volume shrinkage (50%) upon imidization leading to side-wall distortion.
There is a need for positive-working offerings due to a number of potential benefits including greater processing ease, aqueous development, lower defect densities, and the ability to reexpose or rework.
K. Naitoh et al., Journal of Photopolymer Science and Technology 5, 339-342 (1992) and K. Ishii et al., Polymer Preprints Japan 40, 3721 (1991) disclose a photosensitive polyimide system comprising a polyimide resin containing a protected phenol group subject to acid catalyzed deprotection and a source of photogenerated acid. The phenol group is protected by the tetrahydropyranyl group. The protected phenol group is prepared by the reaction of a free phenol group with dihydropyran in the presence of p-toluenesulfonic acid. When this photosensitive polyimide system is exposed to light, acid is generated and, in the light-struck areas, the protected phenol is deprotected and subsequently washed away with a basic developer solution. The non-exposed areas are insoluble and remain behind. These nonexposed areas still contain the thermally-labile tetrahydropyranyl protected phenol.
U.S. Pat. No. 5,212,047 discloses resist compositions that include acid generators and polymers that possess recurring acid-labile groups including alpha alkoxyalkyl carboxylic acid esters and/or hydroxyaryl or hydroxyaralkyl esters of carboxylic acids.
WO 93/09470 describes positive-working photoresists containing a resin such as a polyimide precursor or a soluble polyimide, a photoactivable acid generator, and a solubility enhancer, capable of promoting dissolution of the photoactivated portion of the resist.